Traction control systems include three main components, namely, sensors, controllers, and actuators. The sensors provide the controllers with information about one or more vehicle operating conditions such as: vehicle body speed and acceleration, wheel speeds and accelerations, engine speed and torque, throttle position, and steering wheel angle. The controllers process the sensed information to determine the presence of wheel slip and output a command signal to control the actuators for limiting wheel slip. The actuators include one or more devices for accomplishing such tasks as: braking individual drive wheels, restricting differentiation between the drive wheels, limiting engine output power, and engaging additional drive wheels.
The controllers of most traction control systems include microprocessors that execute programs for evaluating the sensed information by a variety of inferential methods to determine the presence of wheel slip. Some of these systems evaluate a large number of sensed operating conditions to help distinguish wheel slip from other independent operations of the drive wheels. Other of these systems exhibit a wide tolerance for wheel slip to avoid interfering with desirable operations of the drive wheels. The former systems are very costly, and the latter systems are at least partly ineffective for eliminating undesirable wheel slip.
European Patent 252 384 of Audi AG discloses a discovery that occurrences of wheel slip are accompanied by detectable vibrations in vehicle drive trains. The vibrations are within a frequency range between ten to fifteen hertz, and are detectable as both mechanical vibrations in the drive line and rotational speed fluctuations of slipping drive wheels. Although the European patent of Audi AG also contains some general suggestions about incorporating this discovery into traction control systems, these suggestions appear to have been largely overlooked commercially.
For instance, the European patent of Audi AG suggests that in response to the detection of the characteristic vibration of wheel slip, vehicle brakes may be applied, engine power may be reduced, or a combination of these responses may be used to eliminate the wheel slip. In addition, individual drive wheels can be monitored for the characteristic vibrations; and other vehicle operating conditions, such as steering wheel angle, can be sensed to better match the available responses to different vehicle operating conditions.
Although no provisions are made for detecting drive train vibrations as an indication of wheel slip, U.S. Pat. Nos. 4,790,404 of Nissan Motor Company and 4,884,650 of Fuji Jukogyo Kabushiki Kaisha disclose examples of traction control systems that incorporate more than one actuator for limiting wheel slip. In fact, both of these traction control systems incorporate separate actuators for restricting differentiation and reducing engine power. The two systems also distinguish between different vehicle operating conditions for responding to the detection of wheel slip with one or more of the actuators.
The state of the art also includes an example of a traction control system that restricts differentiation as a function of vehicle speed, rather than as a function of an operating condition relating to wheel slip. This is shown in U.S. Pat. No. 4,979,931 of Steyr-Daimler-Puch AG which discloses a differential that is controlled to exhibit a maximum amount of resistance to differentiation when the vehicle is at rest, but the resistance decreases to a minimum amount as the vehicle speed increases from rest to a predetermined speed.